Solute layers and double-diffusive interfaces during the solidification of NH₄Cl-H₂O in rectangular cavities

Flow and temperature fields during the solidification of hypereutectic and hypoeutectic NH₄Cl-H₂O solution in rectangular cavities were measured by a particle image velocimetry(PIV) and a weak perturbation thermocouple network, respectively. Double-diffusive convections caused by the coupling effects of temperature and solute gradients were studied by the experiment. During the solidification of hypereutectic solution, the rejected water near the solidification interface will lead to dilute solute layers and double-diffusive interfaces. As the continued rejection of water, the layer and interface will evolve into instability and a multi-layer and multi-interface structure will be formed. To the hypoeutectic solution, the rejection of NH ₄Cl near the solidification interface will form a dense solute layer. When the thickness of the dense solute layer is large enough, the coupling effects of stabilizing solute gradient and unstable temperature gradient will lead to new solute layers. The solute layers and double-diffusive interfaces will evolve stably and have no breakup of the double-diffusive interfaces during the solidification of hypoeutectic solution.